Conventional blowers disclosed in Japanese Patent Unexamined Publication No. 2002-165477 and Japanese Patent Unexamined Publication No. H10-89674 will be described with reference to FIGS. 11 to 14.
FIG. 11 shows a circuit of conventional brushless DC motor 104 (hereinafter referred to as motor 104). A direct-current voltage that is obtained by rectifying alternating-current voltage 101 is applied to motor 104 via inverter 105. The above circuit has current detector 106, position sensor 107 of a rotor, and control device 110. Control device 110 has number of revolutions detecting unit 111, current detecting unit 112, air-volume calculating unit 113, and speed control unit 114 as shown.
FIG. 12 shows a circuit of speed control unit 114. Air-volume calculating unit 113 calculates the target number of revolutions of motor 104 for attaining a target air volume. Speed control unit 114 generates a PWM signal that has an output voltage for attaining a target number of revolutions. Inverter 105 performs PWM driving of motor 104.
When motor 104 is driven with a constant voltage, as shown in FIG. 13, air volume changes greatly with little change in number of revolutions. Accordingly, if pressure loss changes slightly under the influence of external wind pressure, etc., the air volume will change greatly. The present invention provides a blower that realizes constant air volume control even when a pressure loss changes.